1. Field of the Invention
The present invention relates to a thin film magnetic head, a magnetic recording device using the same and a method for manufacturing the same, and particularly to improvement of a write element provided in a thin film magnetic head.
2. Discussion of Background
In recent years, the improvement in performance of a thin film magnetic head is demanded with the improvement in areal recording density of a hard disk device. The improvement in performance of a thin film magnetic head must be achieved in two aspects. One aspect is the improvement in performance of a read element, and the other is the improvement in performance of a write element.
The performance of a read element has been remarkably improved by development and practical use of a GMR (giant magnetoresistive) head provided with a spin valve film (SV film) or a ferromagnetic tunnel junction. Recently, this trend is so vigorous as to exceed a areal recording density of 100 Gb/p.
On the other hand, the improvement in performance of a write element has various problems to be solved as described below.
First, since a thin film magnetic head is used as a component of a magnetic recording device in a computer, it is demanded to be excellent in high-frequency characteristic and suitable for a high-speed data transfer rate. The high-frequency characteristic of a thin film magnetic head is determined by the structure of yokes and coils to form a write element. From such a view point, various prior arts have been proposed up to now.
For example, U.S. Pat. No. 6,043,959 discloses a technique in which a second yoke (upper yoke) is made flat to reduce the mutual inductance of coils and thus improve a high-frequency characteristic. U.S. Pat. No. 6,259,583B1 discloses a structure in which high-permeability and low-anisotropy layers, and non-magnetic layers are alternately stacked to form a second flat yoke.
A flat pole structure as disclosed in the above-mentioned prior arts is defined by photolithography, and a submicron process through a semiconductor process technique on the pole portion is necessary to achieve a narrow-track structure with an enhanced recording density. However, this submicron process is accompanied by the problems as described below.
First, the narrower the structure of a pole portion is made in a track structure, the more the pole portion is liable to cause a magnetic saturation, with degradation in a write ability. Thus a magnetic material with a high saturation magnetic flux density (hereinafter, referred to as an HiBs material) is needed to make the pole portion.
As HiBs materials, there are known FeN, CoFeN, NiFe, CoNiFe and the like. Among them, FeN, CoFeN and the like show a high saturation magnetic flux density of 2.4 T, for example, but they are difficult to form a pattern by plating, and consequently it becomes necessary to form a film of the material by sputtering and subsequently to pattern the film by ion milling. In case of a sputtering film as thick as 0.2 μm or more, accurate control over a track width of 0.2 μm or less, however, is very difficult, concerned with a mask made of photoresist or a mask formed of a magnetic film to form an upper pole.
On the other hand, NiFe, CoNiFe and the like can be easily patterned by plating. And NiFe provides a saturation magnetic flux density of 1.5 T to 1.6 T by increasing Fe in a composition ratio of Fe to Ni. Additionally NiFe is easy to control the composition ratio.
For a areal recording density of 80 to 100 Gb/p, the track width gets as small as 0.1 to 0.2 μm, demanding a saturation magnetic flux density as high as 2.3 to 2.4 T, and NiFe cannot satisfy the demand. For a plating method, CoNiFe is suitable but CoNiFe is as low as 1.8 T or so in saturation magnetic flux density and cannot satisfy the high saturation magnetic flux density of 2.3 to 2.4 T required for a small track width of 0.1 to 0.2 μm.
Thus it has been usual that a seed film to be a plating ground is made of CoFe which is 2.4 T in saturation magnetic flux density, and thereon is deposited a plating film of CoNiFe which is 2.3 T in saturation magnetic flux density, for example.
In case of forming, for example, the upper pole by the above-mentioned technique, it is necessary to use the upper pole as a mask and thus trim the seed film below the upper pole by ion beam or the like in order to achieve a required narrow track width in the upper pole. However, the seed film is, for example, a sputtering film of CoFe, and thus is very difficult to trimmed by ion beam. Due to this, in case of trimming a lower pole using the upper pole as a mask, the upper pole greatly reduces in film thickness. For example, the upper pole that has been formed as a plating film of 3 to 3.5 μm thick reduces as thin as 1.0 μm. The upper pole having such a thin film thickness causes a magnetic saturation in a write operation, with considerable degradation in an over-write characteristic.
And since it is necessary to trim the upper pole to a very small width of 0.1 to 0.2 μm by means of ion milling, ion beams need to be applied at a large angle. Due to this, a part closer to the tip of the upper pole is more trimmed and therefore the upper pole is formed into the shape of a triangle or a trapezoid. Thus the upper pole reduces in volume and the reduction in volume increases a risk of a magnetic saturation.
Next, in case of trimming a pole, a trimming mask is deposited so as to surround an upper yoke portion and cover a coil portion, not to cover the upper yoke portion and the upper pole. The reason is that it has been thought that covering the whole of the upper yoke portion and the upper pole connected thereto causes a side wall at the edge of the mask pattern and the side wall deposited to the pole causes a side write phenomenon, side erase phenomenon or the like.
As the upper yoke portion is not covered with a mask as described above, a flare portion, which increases gradually in width from the upper pole to a wide portion of the upper yoke portion, is trimmed by ion beam, so that the flare point, at which the upper yoke portion begins to increase in width, backs away from the air bearing surface (hereinafter, referred to as ABS). This also reduces the magnetic volume, with degradation in the over-write characteristic.
Generally, the closer the flare point, at which the flare portion begins to increase in width, is to the ABS, the more excellent over-write characteristic is obtained. The flare point must be made close to the ABS, especially in the case of the small track width of 0.2 μm or less. In the conventional trimming method, the flare point recedes not only for the above-mentioned reason, but also for the following reason.
That is to say, as a trimming mask is deposited so as to surround the upper pole portion and cover the coil portion, not to cover the upper yoke portion and the upper pole, metal particles scattered by trimming the lower pole by ion beams deposit on the side wall faces of the upper pole. To obtain a prescribed track width, the deposit film must be removed. To remove the deposit film, ion beams must be applied at a large angle of 50 to 75 degrees. This ion beam irradiation at a large angle narrows the upper pole. Furthermore, the pole is narrowed to have a taper angle making the width gradually smaller from the flare point toward the ABS, causing a problem that the track width varies according to individual thin film magnetic heads.
And while a narrow-track structure might be achieved by applying a semiconductor process technique on a flat pole structure to perform a submicron process on the pole portion, the surface of the flare portion expanding in width from the pole portion toward the yoke portion forms the same plane as the surfaces of the pole portion and yoke portion, causing problems that, in a write operation, the magnetic flux leaked from a side of the flare portion might erase a magnetic record on an adjacent track in a magnetic recording medium (side erase phenomenon), give a magnetic record to an adjacent track in a magnetic recording medium (side write phenomenon), or the like. Due to these problems, it is difficult to perform an accurate track control of 0.2 μm or less, and consequently it is impossible to achieve a high areal recording density of 100 Gb/p or more.
Next, it is known that in a thin film magnetic head of this type, the shorter the yoke length YL from the back gap to the pole portion is, the more excellent high-frequency characteristic is obtained. In order to shorten the yoke length, it is necessary to reduce the number of turns of a coil positioned between the back gap and the pole portion or to reduce the width of the coil without reducing the number of turns.
As the number of turns of a coil is determined by a magneto motive force required, however, reducing the number of coil turns to shorten the yoke length YL has a limit.
On the other hand, in case of reducing the width of a coil without reducing the number of coil turns, the electric resistance of the coil increases, so a temperature rise due to heat generation in a write operation increases. When the temperature rise increases, the upper pole portion and the lower pole portion thermally expand to cause a thermal protrusion that the pole portion swells on the ABS side. When a thermal protrusion occurs, the part where the thermal protrusion has occurred comes into contact with a magnetic recording medium in write and read operations, causing head crash, damage or destruction of a magnetic record on the magnetic recording medium. Consequently, a thermal protrusion must be strictly avoided. If it is impossible to avoid a thermal protrusion, the floating height of a thin film magnetic head must be increased after all, which makes it impossible to meet a demand for a low floating height for a high recording density.